The origins of the Parthian dynasty lie with a tribe of nomads, the Parni, in the steppes near the Caspian. After gradually infiltrating to the south, they overthrow the Seleucids and take power as a royal house in Parthia in about 247 BC. The founder of their line is Arsaces I, and the dynasty is sometimes known as Arsacid.

The Parthians never lose touch with their origins as horsemen of the steppes, and their brilliance in fighting from the saddle is a large part of their fame. It brings them a great victory over the Romans at Carrhae in 53 BC. The 'Parthian shot', in which a horseman fires an arrow over the rump of the horse as he gallops away, becomes a favourite image of the ancient world.

An agreed boundary with the Parthians is one of the achievements of the peaceful foreign policy of the emperor Augustus. He even recovers for Rome the imperial standards captured by the Parthians at Carrhae, the loss of which has been a cause of deep shame. Negotiations result in the Parthians recognizing Roman sovereignty over Armenia, while Rome agrees not to challenge Parthian rule in Mesopotamia east of the Euphrates.

These friendly arrangements do not prevent Rome from meddling in the affairs of the Parthian royal dynasty by underhand means, in the extraordinary affair of an Italian slave girl, Musa.

Pressure from the east: 1st century BC - 1st century AD

While engaged in the evenly matched tussle with Rome in the west, the Parthians are subject to much more relentless pressure from the east. Just as the Parthians themselves moved down from the steppes into Persia, nomadic tribes from north of the Himalayas are now pressing on the eastern part of the empire. By the 1st century BC the Yueqi are settled in Bactria.

This pressure from the east, combined with the lush appeal of Mesopotamia, has the effect of transferring the centre of Parthian rule westwards. By the 1st century BC they are developing Ctesiphon as their capital, on the opposite bank of the Tigris from the Greek city of Seleucia.

Decline of Parthia: 2nd - 3rd century AD

On several occasions during the 2nd century the Romans invade Parthia, sometimes even reaching Ctesiphon and beyond. They are never able to hold for long any territory which they gain beyond the Euphrates, but their incursions weaken the Parthian royal dynasty.

In keeping with their nomadic origins, the Parthians rule in a feudal fashion - as leaders of a loose hierarchy of powerful local dynasties. One such dynasty, that of the Sassanians, brings the Parthian empire to an end. Repeating a pattern eight centuries old (when Cyrus overthrew the Medes), the rebellious feudal vassal comes from the most ancient land of Persia, the kingdom of Fars, known at this time by the Greek name of Persis.

The Sassanians: 3rd - 7th century AD

The founder of the Sassanian dynasty, Ardashir, has strong links with the ancient Persian religion. His father is in charge of a temple to Zoroaster in the region of the ruined Persepolis before he kills the local ruler and takes his place. Ardashir inherits this petty kingdom and enlarges it - by defeating and killing local princes - until he is in a position to be crowned king of Fars in about 208.

A continuous process of slow expansion, at the expense of the Parthians, brings him to Ctesiphon. He enters the Parthian capital in triumph in about 224 and is crowned 'king of kings'. The new king is proud of one particular ancestor, Sassan; his dynasty becomes known as Sassanian.

Near Persepolis, at Naqsh-e-Rustam, Ardashir commissions a great relief sculpted high in the rock face. It depicts him on horseback, with a dead Parthian beneath his horse's hooves, while he receives the royal crown from Ahura Mazda.

With the restoration of the first authentically Persian dynasty since the Achaemenids, the cult of Ahura Mazda becomes again the official state religion. There is now a ritual hierarchy throughout the empire, with chief priests for each major district and a supreme priest wielding overall authority.

In the following centuries the Sassanian empire is at its greatest extent in two periods: under Ardashir's son Shapur, when Antioch is captured and the Roman emperor Valerian taken prisoner (in 260); and in the time of Khosrau I, who raids into Byzantine Syria, again takes Antioch (in 540) and carries off its famous craftsmen to work on his palace at Ctesiphon (famous for its spectacular Spring Carpet). In both reigns the empire includes territories across the Persian Gulf, in Arabia.

But though there may be brief triumphs, as in the double capture of Antioch or similar Roman successes at Ctesiphon, the overall effect of this long contest between Persia and Rome (or Byzantium) is debilitating to both.

Byzantium and Persia: 6th - 7th century

The final and most destructive chapter in the rivalry between the Byzantine empire and Persia begins in an improbable way. In591 both emperors find themselves fighting on the same side.

Khosrau II has fled from Persia after the murder of his father. He enlists the support of the Byzantine emperor, Maurice, who marches east to restore Khosrau to his inheritance - in return for some useful territorial concessions in Armenia and Mesopotamia. The result is peace between the two sides until 602, when Maurice is murdered in a Byzantine upheaval. Khosrau, seeing his own opportunity, moves to avenge his friend's death. In the next few years the Persians devastate the Byzantine cities of the Middle East.

The first Christian city to fall to Khosrau's armies is Antioch, in 611. Damascus follows in 613. In the spring of 614 a Persian army enters Palestine and moves through the countryside, burning churches. Only the church built by St Helena in Bethlehem is spared; the Persians recognize themselves in the costumes of the Magi, seen bringing their gifts to the infant Jesus in a mosaic above the entrance.

The army reaches Jerusalem in April. The Patriarch urges the inhabitants to surrender, so as to avoid bloodshed, but they resist for a month. When the city falls, it is said that some 60,000 Christians are massacred and another 35,000 sold into slavery.

From the point of view of the Christian hierarchy, far away in Constantinople, the Persians commit one even greater affront. After sacking Jerusalem, they carry off to Ctesiphon the most holy relic of Christendom, the True Cross of Christ.

Its restoration to Jerusalem becomes an urgent matter of state.

Recovering the relic: 622-629

Under the emperor Heraclius, Byzantium has been quietly regaining its strength. In622 Heraclius feels ready to take the field against the Persians. His successes are as rapid and spectacular as the reverses of the previous decade. By 624 he has swept through Asia Minor and Armenia to reach Azerbaijan, to the north of Persia between the Black Sea and the Caspian.

Here, as if avenging the violation of the True Cross, he destroys one of the most sacred fire temples of Zoroastrianism.

In the next few years the swings of fortune become even more extreme. In 626 a Persian army reaches the Bosphorus, but fails to cross the water to support a siege of Constantinople's massive walls by a barbarian horde of Slavs and Avars. In 627 a Byzantine army under Heraclius penetrates Mesopotamia far enough to defeat the Persians at Nineveh and destroy Khosrau's palace at Ctesiphon.

From a position of strength Heraclius negotiates the return of the True Cross. He takes it back to be displayed in Constantinople, and then personally returns it, in 629, to the Holy Sepulchre in Jerusalem. But the relic proves powerless against the next threat to Jerusalem in 638.